This invention relates to magnetic tunnel junction (MTJ) devices for memory, recording head and external magnetic field-sensing applications. More particularly the invention relates to a MTJ device that uses an improved insulating tunnel barrier that improves the properties of the MTJ.
A magnetic tunnel junction (MTJ) is comprised of two layers of ferromagnetic material separated by a thin insulating tunnel barrier layer. The insulating layer is sufficiently thin that quantum-mechanical tunneling of the charge carriers occurs between the ferromagnetic electrodes. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetic moments (magnetization directions) of the two ferromagnetic layers. The two ferromagnetic layers are designed to have different responses to magnetic fields so that the relative orientation of their moments can be varied with an external magnetic field. The MTJ is usable as a memory cell in a nonvolatile magnetic random access memory (MRAM) array, as described in IBM""s U.S. Pat. No. 5,640,343, and as a magnetic field sensor, such as a magnetoresistive read head in a magnetic recording disk drive, as described in IBM""s U.S. Pat. No. 5,729,410.
FIG. 1 illustrates a cross-section of a conventional MTJ device. The MTJ 10 includes a bottom xe2x80x9cfixedxe2x80x9d ferromagnetic (FM) layer 18 as the bottom magnetic electrode, an insulating tunnel barrier layer 20, and a top xe2x80x9cfreexe2x80x9d FM layer 32 as the top magnetic electrode. The MTJ 10 has bottom and top electrical leads, 12, 14, respectively, with the bottom lead 12 being formed on a suitable substrate. The FM layer 18 is called the xe2x80x9cfixedxe2x80x9d layer because it is formed of a high-coercivity material whose magnetic moment (magnetization direction) is prevented from rotation in the presence of applied magnetic fields in the desired range of interest for the MTJ device, i.e., the magnetic field caused by the write current applied to the memory cell from the read/write circuitry of the MRAM or the magnetic field from the recorded magnetic layer in a magnetic recording disk. The magnetic moment of FM layer 18 can also be fixed by being exchange coupled to an antiferromagnetic layer. The fixed FM layer may also be the laminated or antiparallel (AP) pinned type of structure, as described in IBM""s U.S. Pat. No. 5,465,185. The magnetic moment of the free FM layer 32 is not fixed, and is thus free to rotate in the presence of an applied magnetic field in the range of interest. In the absence of an applied magnetic field the moments of the FM layers 18 and 32 are aligned generally parallel (or antiparallel) in a MTJ memory cell and generally perpendicular in a MTJ magnetoresistive read head. The relative orientation of the magnetic moments of the FM layers 18, 32 affects the tunneling current and thus the electrical resistance of the MTJ device.
An important metric for MTJ device applications is the signal-to-noise ratio (SNR). The magnitude of the signal is dependent upon the magnetoresistance or MR (xcex94R/R) exhibited by the device. The magnetoresistance (MR) of a MTJ device is also referred to as the tunneling magnetoresistance (TMR). The signal is given by iB xcex94R, which is the bias current (iB) passing through the MTJ device (assuming a constant current is used to detect the signal) times the resistance change (xcex94R) of the device. However, the noise exhibited by the MTJ device is determined, in large part, by the resistance R of the device. Thus to obtain the maximum SNR for constant power used to sense the device the resistance (R) of the device must be small and the change in resistance (xcex94R) of the device large.
The resistance of a MTJ device is largely determined by the resistance of the insulating tunnel barrier layer for a device of given dimensions since the resistance of the electrical leads and the ferromagnetic layers contribute little to the resistance. Moreover, because the sense current passes perpendicularly through the ferromagnetic layers and the tunnel barrier layer, the resistance R of a MTJ device increases inversely with the area A of the device. The requirement for low resistance MTJ devices, coupled with the inverse relationship of resistance with area, is especially troublesome because an additional requirement for MTJ device applications is small area. For an MRAM the density of MTJ memory cells in the array depends on small area MTJs, and for a read head high storage density requires small data trackwidth on the disk, which requires a small area MTJ read head. Since the resistance R of a MTJ device scales inversely with the area A, it is convenient to characterize the resistance of the MTJ device by the product of the resistance R times the area A (RA). Thus RA is independent of the area A of the MTJ device.
In the prior art, the material used for the tunnel barrier layer is aluminum oxide (Al2O3) because such barrier layers can be made very thin and essentially free of pin holes or other defects. For Al2O3 barrier layers it has been found that RA increases exponentially with the thickness of the layer. The thickness of Al2O3 barrier layers can be varied over a sufficient range to vary RA by more than eight orders of magnitude, i.e., from more than 2xc3x97109 xcexa9(xcexcm)2 to as little as 5 xcexa9(xcexcm)2. However, for these lower RA values, the MR is typically reduced, in part because of microscopic pin holes or other defects in the ultra thin tunnel barrier layers needed to obtain these very low RA values. For MRAM applications RA values in the range 500-1000 xcexa9(xcexcm)2 are acceptable, although it would be useful to be able to prepare MTJ memory cells with even lower RA values so that, for example, current could be passed perpendicularly through the MTJ cell to aid in the writing of the cell. Moreover, for scaling to ever higher memory capacities, MRAM cells will need to be shrunk in size, requiring lower RA values so that the resistance of the cell is not too high. More importantly, for MTJ read heads to be competitive in SNR with conventional giant magnetoresistance (GMR) spin-valve read heads, the MTJ heads need to have resistance values comparable to those of GMR heads. Since read heads of sub-micron size are required for high density recording applications, MTJ heads with RA values lower than 5 xcexa9(xcexcm)2 are desirable, which is an RA value less than what can be obtained with Al2O3 tunnel barriers.
Thus it is desirable to develop MTJ devices with lower RA values than can be achieved with MTJ devices that use conventional single-layer Al2O3 tunnel barriers, and where the lower RA values can be achieved without sacrificing high MR.
In IBM""s pending application, Ser. No. 09/552,062 filed Apr. 19, 2000, oxides and nitrides of indium and gallium are suggested as tunnel barrier materials, wherein for a given barrier thickness a lower tunnel resistance results. This application also suggests that a thin layer of aluminum oxide or nitride can be used with the indium or gallium oxide or nitride to form a bilayer tunnel barrier.
R. Jansen et al. (J Appl. Phys., 83, 11 (June 1998)) describe the effect of adding impurities such as Co, Pd, Cu or Ni into the middle of the insulating aluminum oxide barrier to affect the TMR. A single layer of aluminum oxynitride has been proposed as a tunnel barrier layer for reducing the resistance of the MTJ device in PCT application WO 00/04591, published Jan. 27, 2000, and by M. Sharma et al. (Appl. Phys. Lett. 77, 14 (Oct. 2, 2000)).
The literature also describes bilayers or multiple barriers being used in tunneling structures, but all of these are high resistance devices. For example, A. Vedyayev et al. (Phys. Rev. B. 61, 1366 (2000)) discuss theoretically the magnetoresistance of tunnel-valves with P/O and P/O/P barriers, where P is a paramagnetic metal and O is an insulating oxide, in order to describe the experimental results of J. J. Sun et al for the case of Cu/Al2O3 and Cu/Al2O3/Cu (J. Appl. Phys. 85, 5264 (1999)). This work focuses on the decay of magnetoresistance across the P layers and on the formation of quantum-well states in the P layers. In contrast to the present invention, these studies use a single insulating barrier, result in high specific junction resistances (about 1xc3x97108 xcexa9(xcexcm)2), and the P layer decreases the TMR without reducing the resistance. Other studies such as those by De Teresa et al. (JMMM 211, 160 (2000)) have focussed on the dependence of the TMR effect on the magnetic layer/barrier interface, such as Co/Al2O3 vs. Co/SrTiO3. They have used bilayer barriers in those studies, such as SrTiO3/Al2O3, to illustrate the effect of different interfaces on the two magnetic electrodes. P. K. Wong et al. (J. Appl. Phys. 83, 11 (Jun. 1, 1998)) prepared, as shown in Table 1, an MTJ device with a tunnel barrier of a bottom Al2O3/top MgO bilayer formed by sputter depositing Al/Mg in an oxygen atmosphere, but that device resulted in a high RA product of 1.2xc3x97108 xcexa9(xcexcm)2. Again, in contrast to the present invention, these studies concern tunnel barriers with much higher resistance (about 2xc3x97104 xcexa9(xcexcm)2 and higher) and do not address the applicability of bilayer barriers to low-resistance (low RA) magnetic tunnel junction devices.
In addition, other parameters besides the overall MTJ device resistance need to be optimized. These include the magnetic properties of each magnetic electrode, the magnetic coupling between the magnetic electrodes, the electron spin polarization at the interfaces between the magnetic electrodes and the tunnel barrier and the degree of atomic interdiffusion between the magnetic electrodes and the barrier. These parameters influence the magnetic performance of the MTJ device, the magnitude of the TMR, and the dependence of the magnetoresistance on the bias voltage applied to the device. Because each of these parameters is primarily dependent on the nature of the interface region between the barrier and the magnetic electrodes, there is a need for an improved tunnel barrier structure which can improve these characteristics in MTJ devices and which offers more flexibility in obtaining desired MTJ properties.
The invention is a low resistance magnetic tunnel junction device with a bilayer or multilayer insulating tunnel barrier. In one bilayer barrier embodiment the tunnel barrier is a first layer of magnesium oxide on the bottom magnetic electrode and an aluminum oxide layer on the magnesium oxide layer. The bilayer is formed by oxidizing a brayer of Mg/Al. In a second bilayer embodiment the first layer is aluminum nitride and the second layer is aluminum oxide on top of the aluminum nitride first layer, with this bilayer formed by oxidizing a bilayer of AlN/Al. Trilayer barrier embodiments are also possible, such as AlN/Al2O3/AlN, MgO/Al2O3/MgO and Al2O3/MgO/Al2O3 . The resulting magnetic tunnel junction devices have resistance-area values less than 1000 xcexa9(xcexcm)2and preferably in the range of 0.1 to 100 xcexa9(xcexcm)2, making the devices suitable for magnetic read sensors.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the following detailed description taken together with the accompanying figures.